Apparatus and Method for Pressure Fluctuation Insensitive Mass Flow Control

ABSTRACT

A mass flow controller includes a thermal mass flow sensor in combination with a pressure sensor to provide a mass flow controller that is relatively insensitive to fluctuations in input pressure. The pressure sensor and thermal sensor respectively provide signals to an electronic controller indicating the measured inlet flow rate and the pressure within the dead volume. The electronic controller employs the measured pressure to compensate the measured inlet flow rate and to thereby produce a compensated measure of the outlet flow rate, which may be used to operate a mass flow controller control valve.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/873,089,filed Aug. 31, 2010, which is a continuation of U.S. application Ser.No. 11/950,966, filed Dec. 5, 2007, now issued U.S. Pat. No. 7,809,473,entitled “Apparatus and Method for Pressure Fluctuation Insensitive MassFlow Control,” (Attorney Docket No. MKSK-102CN3), which is acontinuation of U.S. application Ser. No. 11/304,456, filed Dec. 15,2005, now issued U.S. Pat. No. 7,424,346, which is a continuation ofU.S. application Ser. No. 10/178,119, filed Jun. 24, 2002, now issuedU.S. Pat. No. 7,136,767, entitled “Apparatus And Method for CalibrationOf Mass Flow Controller,” filed in the name of Jesse Ambrosina, NicholasKottenstette and Ali Shajii (Attorney Docket No. MKS-107). Thisapplication is related to U.S. application Ser. No. 10/178,378, filedJun. 24, 2002, now issued U.S. Pat. No. 6,948,508, entitled “ApparatusAnd Method For Self-Calibration of Mass Flow Controller,” and filed inthe name of Jesse Ambrosina, Nicholas Kottenstette, Ali Shajii andDonald Smith (Attorney Docket No. MKS-108); U.S. Ser. No. 10/178,884,filed Jun. 24, 2002, now issued U.S. Pat. No. 6,810,308, entitled“Apparatus And Method For Mass Flow Controller With Network Access ToDiagnostics” and filed in the name of Jesse Ambrosina, NicholasKottenstette and Ali Shajii (Attorney Docket No. MKS-109); U.S. Ser. No.10/178,586, filed Jun. 24, 2002, now issued U.S. Pat. No. 7,552,015,entitled “Apparatus And Method For Displaying Mass Flow ControllerPressure” and filed in the name of Jesse Ambrosina, NicholasKottenstette and Ali Shajii (Attorney Docket No. MKS-110); U.S. Ser. No.10/178,752, filed Jun. 24, 2002 (abandoned), entitled “Apparatus AndMethod For Dual Processor Mass Flow Controller” and filed in the name ofJesse Ambrosina, Nicholas Kottenstette and Ali Shajii (Attorney DocketNo. MKS-111); U.S. Ser. No. 10/178,810, filed Jun. 24, 2002, now issuedU.S. Pat. No. 7,004,191, entitled “Apparatus And Method For Mass FlowController With Embedded Web Server” and filed in the name of JesseAmbrosina, Nicholas Kottenstette and Ali Shajii (Attorney Docket No.MKS-112); U.S. Ser. No. 10/178,288, filed Jun. 24, 2002 (abandoned),entitled “Apparatus And Method For Mass Flow Controller With On-LineDiagnostics” and filed in the name of Jesse Ambrosina, NicholasKottenstette and Ali Shajii (Attorney Docket No. MKS-113); U.S. Ser. No.10/178,261, filed Jun. 24, 2002, now issued U.S. Pat. No. 6,868,862,entitled “Apparatus And Method For Mass Flow Controller With A PluralityOf Closed Loop Control Code Sets” and filed in the name of JesseAmbrosina, Nicholas Kottenstette and Ali Shajii (Attorney Docket No.MKS-114); assigned to the present assignee, the contents of all of whichare incorporated herein in their entirety by reference. This applicationis also related to U.S. Ser. No. 10/806,974, filed Mar. 23, 2004, nowissued U.S. Pat. No. 6,932,098, entitled “Apparatus And Method ForPressure Fluctuation Insensitive Mass Flow Control” and filed in thename of Ali Shajii, Nicholas Kottenstette, Jesse Ambrosina, Donald K.Smith and William R. Clark (Attorney Docket No. MKS-102CN), which is acontinuation of U.S. Ser. No. 10/178,721, filed Jun. 24, 2002, nowissued U.S. Pat. No. 6,712,084, entitled “Apparatus And Method ForPressure Fluctuation Insensitive Mass Flow Control” and filed in thename of Jesse Ambrosina, Nicholas Kottenstette and Ali Shajii (AttorneyDocket No. MKS-102), the contents of both of which are incorporatedherein in their entirety by reference.

FIELD OF THE INVENTION

The present invention relates to mass flow sensing and control systems.

BACKGROUND OF THE INVENTION

Capillary tube thermal mass flow sensors exploit the fact that heattransfer to a fluid flowing in a laminar tube from the tube walls is afunction of mass flow rate of the fluid, the difference between thefluid temperature and the wall temperature, and the specific heat of thefluid. Mass flow controllers employ a variety of mass flow sensorconfigurations. For example, one type of construction involves astainless steel flow sensor tube with one, and more typically two ormore, resistive elements in thermally conductive contact with the sensortube. The resistive elements are typically composed of a material havinga high temperature coefficient of resistance. Each of the elements canact as a heater, a detector, or both. One or more of the elements isenergized with electrical current to supply heat to the fluid streamthrough the tube. If the heaters are supplied with constant current, therate of fluid mass flow through the tube can be derived from temperaturedifferences in the elements. Fluid mass flow rates can also be derivedby varying the current through the heaters to maintain a constanttemperature profile.

Such thermal mass flow sensors may be attached as a part of a mass flowcontroller, with fluid from the controller's main channel feeding thecapillary tube (also referred to herein as the sensor tube). The portionof the main channel to which the inlet and outlet of the sensor tube areattached is often referred to as the “bypass” of the flow sensor. Manyapplications employ a plurality of mass flow controllers to regulate thesupply of fluid through a supply line, and a plurality of the supplylines may be “tapped off” a main fluid supply line. A sudden change inflow to one of the controller's may create pressure fluctuations at theinlet to one or more of the other controllers tapped off the main supplyline. Such pressure fluctuations create differences between the flowrate at the inlet and outlet of an affected mass flow controller.Because thermal mass flow sensors measure flow at the inlet of a massflow controller, but outlet flow from the controller is the criticalparameter for process control, such inlet/outlet flow discrepancies canlead to significant process control errors.

In a semiconductor processing application, a process tool may include aplurality of chambers with each chamber having one or more mass flowcontrollers controlling the flow of gas into the chamber. Each of themass flow controllers is typically re-calibrated every two weeks. There-calibration process is described, for example, in U.S. Pat. No.6,332,348 B1, issued to Yelverton et al. Dec. 25, 2001, which is herebyincorporated by reference. In the course of such an “In Situ”calibration, conventional methods require a technician to connect a massflow meter in line with each of the mass flow controllers, flow gasthrough the mass flow meter and mass flow controller, compare the massflow controller reading to that of the mass flow meter and adjustcalibration constants, as necessary. Such painstaking operations canrequire a great deal of time and, due to labor costs and theunavailability of process tools, with which the mass flow controllersoperate, can be very costly.

A mass flow sensor that substantially eliminates sensitivity to pressurevariations would therefore be highly desirable. A convenient calibrationmethod and apparatus for mass flow controllers would also be highlydesirable. More flexible access to a mass flow controller would also behighly desirable. Apparatus and method for increasing the controlperformance of a mass flow controller would also be highly desirable.

SUMMARY OF THE INVENTION

In an illustrative embodiment, a mass flow controller in accordance withthe principles of the present invention includes the combination of athermal mass flow sensor and a pressure sensor to provide a mass flowcontroller that is relatively insensitive to fluctuations in inputpressure. The new controller is relatively inexpensive, that is, it doesnot require a pair of expensive, precision, pressure sensors nor anall-stainless steel wetted surface differential sensor. Nevertheless,the new controller is adapted to control fluid flow over a broad rangeof fluid pressures. The new mass flow controller includes a thermal massflow sensor, a pressure sensor, and an electronic controller. Thethermal mass flow sensor is configured to measure the inlet flow of thecontroller. The pressure sensor senses the pressure within the volume inthe channel between the flow sensor's bypass and an outlet controlvalve, which volume will be referred to herein as the “dead volume.” Thepressure sensor and thermal mass flow sensor respectively providesignals to the controller indicating the measured inlet flow rate andthe pressure within the dead volume. A temperature sensor may beemployed to sense the temperature of the fluid within the dead volume.In an illustrative embodiment, the temperature sensor senses thetemperature of the controller's wall, as an approximation of thetemperature of the fluid within the dead volume. The volume of the deadvolume is determined, during manufacturing or a calibration process, forexample, and may be stored or downloaded for use by the electroniccontroller.

The controller employs the measured pressure within the dead volume tocompensate the measured inlet flow rate figure and to thereby produce acompensated measure of the outlet flow rate as a function of themeasured pressure and measure inlet flow rate. This compensated measureof outlet flow rate may be used to operate a mass flow controllercontrol valve. By reading the pressure sensor output over a period oftime, the electronic controller determines the time rate of change ofpressure within the dead volume. Given the dead volume, the temperatureof the fluid within the dead volume, and the input flow rate sensed bythe thermal mass flow sensor, the electronic controller computes thefluid flow rate at the output of the mass flow controller as a functionof these variables. The electronic controller employs this computedoutput fluid flow rate in a closed loop control system to control theopening of the mass flow controller output control valve. In anillustrative embodiment the pressure sensed by the pressure sensor mayalso be displayed, locally (that is, at the pressure sensor) and/orremotely (at a control panel or through a network interface, forexample).

In accordance with another aspect of the principles of the presentinvention, a variable-flow fluid source, a receptacle of known volume,and a pressure differentiator may be used to calibrate a mass flowcontroller. The variable-flow fluid source supplies gas at varying ratesto the mass flow controller being calibrated and at proportional ratesto a receptacle of known volume. A pressure differentiator computes thetime derivative of gas flow into the receptacle of known volume and,from that, the actual flow into the receptacle. Given the actual flow,the proportionate flow into the mass flow controller may be determinedand the flow signal from the mass flow controller correlated to theactual flow. In an illustrative embodiment, a mass flow controllercloses the outlet valve to form a receptacle of known volume (the deadvolume). A pressure sensor located within the dead volume produces asignal that is representative of the pressure within the dead volume.With the outlet valve closed, the flow into the dead volume decreasesexponentially while the pressure increases, until the pressure withinthe dead volume is equal to that at the inlet to the mass flowcontroller. The mass flow controller's electronic controller takes thetime derivative of the pressure at a plurality of times. Given the deadvolume/receptacle volume, the time derivative of the pressure within thedead volume, and the temperature of the gas, the controller computes theflow rate at those sample times. The electronic controller alsocorrelates the flow rates thus computed to the flow readings produced bythe mass flow controller's thermal mass flow sensor, thereby calibratingthe mass flow controller. This operation is self-contained, in that itdoesn't require the use of external mass flow meters or othercalibration devices. Various techniques and mechanisms may be employedto extend the period of time over which flow continues into the deadvolume, thereby permitting the computation of a greater number ofcorrelation, or calibration, points. For example, the outlet valve maybe fully opened before being shut at the beginning of a calibrationprocess or flow restrictors may be inserted at various locations withinthe gas flow path, for example.

In accordance with another aspect of the principles of the presentinvention, a mass flow controller includes an interface that permits anoperator, such as a technician, to conduct diagnostics through anetwork. Such diagnostics may be “active”, “passive” “on-line”,“off-line”, “manual”, or “automatic” or various combinations of theabove. By “active” diagnostics, we mean diagnostics that permit anoperator to change drive signals in addition to, or instead of,monitoring signals. Enabling the use of drive signals permits atechnician to alter a test point setting, to thereby change currentthrough a resistor, for example. The technician may then monitor acorresponding signal, from a current sensor, for example. Or, atechnician may elect to alter the drive signal to a valve actuatordirectly, as opposed to setting a flow set-point and relying upon themass flow controller's electronic controller to adjust the valve drivesignal in the desired manner. Because such alterations present thepotential for creating flow control errors, access to such control maybe limited, through use of passwords and other security measures, forexample, at the network level. The term “passive” diagnostics refers todiagnostics that include monitoring functions, for example. The term“on-line” diagnostics is used to refer to diagnostics that are both realtime and operating concurrently with the mass flow controller's processcontrol operations. The term “off-line” diagnostics refers todiagnostics that, although they may be real time, are not operatingduring a mass flow controller's process control operations. The term“automatic” diagnostics refers to diagnostics including a plurality ofdiagnostic steps, each of which may be active or passive. The term“manual” diagnostics refers to diagnostics that are responsive on a stepby step basis, to an operator's input.

A mass flow controller in accordance with another aspect of theprinciples of the present invention includes a web server that permitsan operator, such as a technician, to interact with the mass flowcontroller from a web-enabled device, such as a workstation, laptopcomputer, or personal digital assistant, for example, over aninterworking network, such as the Internet. The mass flow controller webserver may include web pages that provide manufacturer's part number,specification, installation location, and performance information, forexample. Additionally, diagnostics may be conducted from a web-enableddevice over an interworking network.

In accordance with yet another aspect of the principles of the presentinvention, a mass flow controller may include a pressure display thatdisplays the pressure within the mass flow controller. The display maybe local, that is, directly in contact with or supported by the massflow controller, or the display may be remote, at a gas box controlpanel, for example. In an illustrative embodiment, a pressure sensor ispositioned to measure the pressure within a mass flow controller's deadvolume and that is the pressure that is displayed.

A dual-processor mass flow controller in accordance with the stillanother aspect of the principles of the present invention includes adeterministic processor that performs the mass flow controllers' controlduties and a non-deterministic processor that handles such tasks asproviding a user interface. In an illustrative embodiment, thedeterministic processor is a digital signal processor (DSP).

In accordance with yet another aspect of the principles of the presentinvention, a plurality of executable code sets may be uploaded by a massflow controller's electronic controller. In an illustrative embodiment,a dual processor mass flow controller's non-deterministic processoruploads a plurality of executable code sets for the deterministicprocessor and selects among the code sets those for the deterministicprocessor to execute. Such selection by the non-deterministic processormay enable a form of customization.

These and other advantages of the present disclosure will become moreapparent to those of ordinary skill in the art after having read thefollowing detailed descriptions of the preferred embodiments, which areillustrated in the attached drawing figures. For convenience ofillustration, elements within the Figures may not be drawn to scale.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a system that includes a mass flow sensorin accordance with the principles of the present invention;

FIG. 2 is a sectional view of a mass flow controller that employs a massflow sensor in accordance with the principles of the present invention;

FIG. 3 is a sectional view of an illustrative thermal mass flow sensoras used in conjunction with a pressure sensor to produce a compensatedindication of mass flow through a mass flow controller;

FIG. 4 is a block diagram of the control electronics employed by anillustrative embodiment of a mass flow sensor in accordance with theprinciples of the present invention;

FIG. 5 is a flow chart of the process of compensating a thermal massflow sensor signal in accordance with the principles of the presentinvention;

FIG. 6 is a conceptual block diagram of a web-enabled mass flowcontroller in accordance with the principles of the present invention;

FIG. 7 is a conceptual block diagram of calibrator such as may beemployed with a mass flow controller in accordance with the principlesof the present invention;

FIG. 8 is a block diagram of a self-calibrating mass flow controller inaccordance with the principles of the present invention;

FIG. 9 is a graphical representation of flow and pressure curvescorresponding to the process of calibrating a mass flow controller inaccordance with the principles of the present invention;

FIG. 10 is a conceptual block diagram of a dual processor configurationsuch as may be used in a mass flow controller in accordance with theprinciples of the present invention;

FIG. 11 is a flow chart of the general operation of a mass flowcontroller's non-deterministic processor in accordance with theprinciples of the present invention;

FIGS. 12A and 12B are flow charts of the general operation of a massflow controller's deterministic processor in accordance with theprinciples of the present invention; and

FIGS. 13A through 13E are screen shots of web pages such as may beemployed by a web server embedded within a mass flow controller inaccordance with the principles of the present invention.

DETAILED DESCRIPTION OF DISCLOSURE

A mass flow sensor in accordance with one aspect of the principles ofthe present invention employs a thermal mass flow sensor to sense andprovide a measure of the flow of fluid into an inlet of a fluid flowdevice, such as a mass flow controller. In an illustrative embodiment,the mass flow sensor uses a pressure sensor to compensate the inlet flowmeasure provided by the thermal mass flow sensor to thereby provide anindicator that more accurately reflects the fluid flow at the outlet ofthe associated mass flow controller. A system 100 that benefits from andincludes the use of a mass flow sensor in accordance with the principlesof the present invention is shown in the illustrative block diagram ofFIG. 1.

A plurality of mass flow controllers MFC1, MFC2, . . . MFCn receive gasfrom main gas supply lines 102,103. The mass flow controllers, MFC1,MFC2, . . . MFCn are respectively connected through inlet supply lines104, 106, . . . 109 to a main gas supply line 102,103 and throughrespective outlet supply lines 110, 112, . . . 115 to chambers C1, C2, .. . Cn. In this illustrative embodiment, the term “chamber” is used in abroad sense, and each of the chambers may be used for any of a varietyof applications, including, but not limited to, reactions involved inthe production of semiconductor components. Generally, users of thechambers are interested in knowing and controlling the amount of eachgas supplied to each of the chambers C1, C2, . . . Cn. Each chamber C1,C2, . . . Cn may also include one or more additional inlet lines for thesupply of other types of gases. Outflow from the chambers may be routedthrough lines (not shown) for recycling or disposal.

The mass flow controllers, MFC1, MFC2, . . . MFCn, include respectivemass flow sensors MFS1, MFS2, . . . MFSn, electronic controllers EC1,EC2, . . . ECn and outlet control valves OCV1, OCV2, . . . OCVn. Atleast one of the mass flow sensors is, and, for ease of description,assume all are, compensated mass flow sensors in accordance with theprinciples of the present invention. Each mass flow sensor senses themass of gas flowing into the mass flow controller and provides a signalindicative of the sensed value to a corresponding electronic controller.The electronic controller compares the indication of mass flow asindicated by the sensed value provided by the mass flow sensor to a setpoint and operates the outlet control valve to minimize any differencebetween the set point and the sensed value provided by the mass flowsensor. Typically, the set point may be entered manually, at the massflow controller, or downloaded to the mass flow controller. The setpointmay be adjusted, as warranted, through the intervention of a humanoperator or automatic control system. Each of the inlet supply lines104, 106, . . . 109 may be of a different gauge, and/or may handle anyof a variety of flow rates into the mass flow controller. In accordancewith one aspect of the principles of the present invention, a singleelectronic controller, such as electronic controller EC1, may be linkedto and operate a plurality of mass flow sensor/outlet control valvecombinations. That is, for example, any number of the illustratedelectronic controllers EC2 through ECn may be eliminated, with thecorresponding mass flow sensors and outlet control valves linked to theelectronic controller EC1 for operation.

An abrupt change of flow rate, due to a change in set point for example,into any of the mass flow controllers may be reflected as an abruptpressure change at the inlet of one or more of the other mass flowcontrollers. This unwanted side effect may be more pronounced in arelatively low flow rate mass flow controller if the abrupt changeoccurs in a high flow-rate mass flow controller. Because the mass flowsensors in this illustrative embodiment are thermal mass flow sensorspositioned to sense flow in the mass flow controller at the inlet to themass flow controller, the mass flow sensed by the thermal mass flowsensor may not accurately reflect the flow at the outlet of thecontroller. In order to compensate for this discrepancy, a mass flowsensor in accordance with one aspect of the principles of the presentinvention includes a pressure sensor positioned to provide an indicationof the pressure within the volume between the inlet and outlet of themass flow controller. In an illustrative embodiment, the pressure sensoris located in the “dead volume” between the thermal mass flow sensor'sbypass and the outlet control valve. An electronic controller employsthe indication of pressure provided by the pressure sensor to compensatethe measure of mass flow provided by the thermal mass flow sensor. Theresult, a compensated mass flow indication, more accurately reflects theflow at the outlet of the mass flow controller and, consequently, thisindication may be employed to advantage by a mass flow controller in theoperation of its outlet control valve. A display may be included todisplay the sensed pressure. The display may be local, attached to orsupported by the mass flow controller, or it may be remote, at a gas boxcontrol panel, for example, connected to the mass flow controllerthrough a data link.

In a semiconductor processing application, a process tool may include aplurality of chambers with each chamber having a plurality of mass flowcontrollers respectively controlling the flow of constituent gases intothe chamber. Usually, each of the mass flow controllers of the prior artis typically re-calibrated every two weeks. The re-calibration processis described, for example, in U.S. Pat. No. 6,332,348 B1, issued toYelverton et al. Dec. 25, 2001, which is hereby incorporated byreference. In the course of such an “In Situ” calibration, conventionalmethods require a technician to (1) connect a mass flow meter in linewith each of the mass flow controllers, (2) flow gas through the massflow meter and mass flow controller, (3) compare the mass flowcontroller reading to that of the mass flow meter and (4) adjustcalibration constants, as necessary. Such painstaking operations canrequire a great deal of time and, due to labor costs and theunavailability of process tools with which the mass flow controllersoperate, can be very costly. In an illustrative embodiment described ingreater detail in the discussion related to FIG. 7, a mass flowcontroller in accordance with the principles of the present inventionincludes a self-calibrating mechanism that substantially eliminates suchtedious and costly chores.

The sectional view of FIG. 2 provides an illustration of a mass flowcontroller 200 that employs a mass flow sensor 202 in accordance withone aspect of the principles of the present invention. The mass flowsensor 202 includes a thermal mass flow sensor 204, a pressure sensor206, a temperature sensor 208 and an electronic controller 210. Alaminar flow element 212 establishes a pressure drop across thecapillary tube of the thermal mass flow sensor 204, as will be describedin greater detail in the discussion related to FIG. 3. In operation, afluid that is introduced to the mass flow controller 200 through theinlet 214 proceeds through the bypass channel 216 containing the laminarflow element 212. A relatively small amount of the fluid is divertedthrough the thermal mass flow sensor 204 and re-enters the bypasschannel 216 downstream of the laminar flow element 212. The electroniccontroller 210 provides a signal to the control valve actuator 218 tothereby operate the outlet control valve 220 in a way that provides acontrolled mass flow of fluid to the outlet 222. The pressure sensor 206senses the pressure within the volume within the bypass channel 216between the laminar flow element 212 and the outlet control valve 220,referred to herein as the “dead volume” As will be described in greaterdetail in the discussion related to FIG. 5, the electronic controller210 employs the pressure sensed within the dead volume by the sensor 206to compensate the inlet flow rate sensed by the thermal mass flow sensor204. This compensated inlet flow rate figure more closely reflects theoutlet flow rate, which is the ultimate target of control. Inparticular, a mass flow sensor in accordance with one aspect of theprinciples of the present invention is a combination sensor that employsthe time rate of change of pressure within a known volume to provide aprecise measure of mass flow during pressure transients and a thermalmass flow sensor that may be “corrected” using the pressure-derived massflow measurement. Both the thermally-sensed and pressure-derived massflow measurements are available for processing. The temperature sensor208 senses the temperature of the fluid within the dead volume. In anillustrative embodiment, the temperature sensor 208 senses thetemperature of a wall of the controller, as an approximation of thetemperature of the fluid within the dead volume 216 a.

The volume of the dead volume is determined, during manufacturing or acalibration process, for example, and may be stored or downloaded foruse by the electronic controller 210. By taking sequential readings fromthe pressure sensor 206 output and operating on that data, theelectronic controller 210 determines the time rate of change of pressurewithin the dead volume. Given the dead volume, the temperature of thefluid within the dead volume, the input flow rate sensed by the thermalmass flow sensor 204, and the time rate of change of pressure within thedead volume, the electronic controller 210 approximates the fluid flowrate at the output 222 of the mass flow controller 200. As previouslynoted, this approximation may also be viewed as compensating the massflow rate figure produced by the thermal mass flow sensor 204. Theelectronic controller 210 employs this computed output fluid flow ratein a closed loop control system to control the opening of the mass flowcontroller outlet control valve 220. In an illustrative embodiment thevalue of the pressure sensed by the pressure sensor 206 may also bedisplayed, locally (that is, at the pressure sensor) and/or remotely (ata control panel or through a network interface, for example). In aself-calibrating process described hereinafter in the discussion relatedto FIG. 7, the electronic controller 210 may take the time derivative ofthe pressure signal when the flow rate varies in the mass flowcontroller and thereby derive the actual flow rate into the mass flowcontroller. The actual flow rate may then be used to calibrate the massflow controller.

The sectional view of FIG. 3 provides a more detailed view of a thermalmass flow sensor 204, such as may be employed in conjunction with apressure sensor to produce a compensated mass flow indication that is,in a digital implementation, a multi-bit digital value. The multi-bitdigital value provides a closer approximation to the actual mass flow atthe outlet of a mass flow controller than an uncompensated mass flowsensor would, particularly during pressure transients on the mass flowcontroller inlet lines. The thermal mass flow sensor 204 includeslaminar flow element 212, which rests within the bypass channel 216 andprovides a pressure drop across the bypass channel 216 for the thermalmass flow sensor 204 and drives a portion of the gas through the sensorcapillary tube 320 of the thermal mass flow sensor 204. The mass flowsensor 202 includes circuitry that senses the rate of flow of gasthrough the controller 100, 200 and controls operation of the controlvalve 220 accordingly. The thermal mass flow sensor assembly 204 isattached to a wall 322 of the mass flow controller that forms a boundaryof the bypass channel 216. Input 324 and output 326 apertures in thewall 322 provide access to the mass flow sensor assembly 204 for a gastraveling through the mass flow controller and it is the portion of thispassageway between the input and output that typically defines thebypass channel. In this illustrative embodiment the mass flow sensorassembly 204 includes a baseplate 328 for attachment to the wall 322.The baseplate 328 may de attached to the wall and to the remainder ofthe sensor assembly using threaded hole and mating bolt combinations,for example. Input 330 and output 332 legs of the sensor tube 320 extendthrough respective input 334 and output 336 apertures of the baseplate328 and, through apertures 324 and 326, the mass flow controller wall322.

The mass flow sensor assembly preferably includes top 338 and bottom 340sections that, when joined, form a thermal clamp 341 that holds bothends of the sensor tube 320 active area (that is, the area defined bythe extremes of resistive elements in thermal contact with the sensortube) at substantially the same temperature. The thermal clamp alsoforms a chamber 342 around the active area of the sensor tube 320. Thatis, the segment of the mass flow sensor tube within the chamber 342 isin thermal communication with two or more resistive elements 344, 346,each of which may act as a heater, a detector, or both. One or more ofthe elements is energized with electrical current to supply heat to thefluid as it streams through the tube 320. The thermal clamp 341, whichis typically fabricated from a material characterized by a high thermalconductivity relative to the thermal conductivity of the sensor tube,makes good thermally conductive contact with the portion of the sensortube just downstream from the resistive element 344 and with the portionof the sensor tube just upstream from the resistive element 346. Thethermal clamp thereby encloses and protects the resistive element 344and 346 and the sensor tube 320. Additionally, the thermal clamp 341thermally “anchors” those portions of the sensor tube with which itmakes contact at, or near, the ambient temperature. In order toeliminate even minute errors due to temperature differentials, thesensor tube may be moved within the thermal clamp to insure that anydifference between the resistance of the two coils is due to fluid flowthrough the sensor tube; not to temperature gradients imposed upon thecoils from the environment.

In this illustrative embodiment, each of the resistive elements 344 and346 includes a thermally sensitive resistive conductor that is woundaround a respective portion of the sensor tube 320. Each of theresistive elements extends along respective portions of the sensor tube320 along an axis defined by the operational segment of the sensor tube320. Downstream resistive element 346 is disposed downstream of theresistive element 344. The elements abut one another or are separated bya small gap for manufacturing convenience and are preferablyelectrically connected at the center of the tube. Each resistive element344, 346 provides an electrical resistance that varies as a function ofits temperature. The temperature of each resistive element varies as afunction of the electrical current flowing through its resistiveconductor and the mass flow rate within the sensor tube 320. In thisway, each of the resistive elements 344, 346 operates as both a heaterand a sensor. That is, the element acts as a heater that generates heatas a function of the current through the element and, at the same time,the element acts as a sensor, allowing the temperature of the element tobe measured as a function of its electrical resistance. The thermal massflow sensor 204 may employ any of a variety of electronic circuits,typically in a Wheatstone bridge arrangement, to apply energy to theresistive elements 346 and 344, to measure the temperature dependentresistance changes in the element and, thereby, the mass flow rate offluid passing through the sensor tube 320. Circuits employed for thispurpose are disclosed, for example, in U.S. Pat. No. 5,461,913, issuedto Hinkle et al and U.S. Pat. No. 5,410,912 issued to Suzuki, both ofwhich are hereby incorporated by reference in their entirety.

In operation, fluid flows from the inlet 214 to the outlet 222 and aportion of the fluid flows through the restrictive laminar flow element212. The remaining and proportional amount of fluid flows through thesensor tube 320. The circuit (not shown here) causes an electricalcurrent to flow through the resistive elements 344 and 346 so that theresistive elements 344 and 346 generate and apply heat to the sensortube 320 and, thereby, to the fluid flowing through the sensor tube 320.Because the upstream resistive element 346 transfers heat to the fluidbefore the fluid reaches the portion of the sensor tube 320 enclosed bythe downstream resistive element 344, the fluid conducts more heat awayfrom the upstream resistive element 346 than it does from the downstreamresistive element 344. The difference in the amount of heat conductedaway from the two resistive elements is proportional to the mass flowrate of fluid within the sensor tube and, by extension, the total massflow rate through the mass flow rate controller 200 from the input port214 through the output port 222. The circuit measures this difference bysensing the respective electrical resistances of resistive elements 344,346 and generates an output signal that is representative of the massflow rate through the sensor tube 320.

The conceptual block diagram of FIG. 4 illustrates the architecture ofan electronic controller 400 such as may be used in a mass flow sensorin accordance with the principles of the present invention. In thisillustrative embodiment, the controller 400 includes sensor 402 andactuator 404 interfaces. Among the sensor interfaces 402, a flow sensorinterface 408 operates in conjunction with a mass flow sensor to producea digital representation of the rate of mass flow into an associatedmass flow controller. The controller 400 may include various othersensor interfaces, such as a pressure sensor interface 410 or atemperature sensor interface 411. One or more actuator drivers atinterface 412 are employed by the controller 400 at the actuator 404 tocontrol, for example, the opening of an associated mass flowcontroller's output control valve. The actuator may be any type ofactuator, such as, for example, a current-driven solenoid or avoltage-driven piezo-electric actuator.

The controller 400 operates in conjunction with a mass flow controllerto produce a digital representation of the rate of mass flow into anassociated mass flow controller. A thermal mass flow controller, such asdescribed in the discussion related to FIG. 3, may be employed toproduce the mass flow measurement. The controller 400 may employ apressure sensor interface 410 to monitor the pressure of fluid within anassociated mass flow controller. In an illustrative embodiment, apressure sensor, such as the pressure sensor 206 of FIG. 2, provides ameasure of the pressure within the mass flow controller. Morespecifically, in this illustrative embodiment, the sensor measures thepressure within dead volume of the mass flow controller. In anillustrative embodiment, the mass flow controller pressure thus measuredmay be displayed, at the pressure sensor 206 or at the controllerhousing, for example, or some other location.

The controller 400 may convert the pressure measurement to digital formand employ it in analysis or other functions. For example, if the massflow controller employs a thermal mass flow sensor, the controller 400may use the mass flow controller pressure measurement to compensate forinlet pressure transients. Although a temperature sensor interface maybe used to obtain a temperature reading from a temperature sensorattached, for example, to the wall of a mass flow controller, a separatetemperature sensor may not be required for each mass flow controller.For example, mass flow controllers are often employed, as described ingreater detail in the discussion related to FIG. 1, in conjunction witha semiconductor processing tool that includes a number of mass flowcontrollers and other devices that are all linked to a controller, suchas a workstation. The processing tool is operated within a carefullycontrolled environment that features a relatively stable temperature.Because the temperature of the fluid within the mass flow controller isvery nearly equal to that of the wall of the enclosure and the wall ofthe enclosure is very nearly the temperature of the room within whichthe tool is housed, a temperature measurement from, for example, theworkstation that controls the tool, may provide a sufficiently accurateestimate of the gas temperature within the mass flow controller.Consequently, in addition to, or instead of, employing a separatetemperature sensor on each mass flow controller, the temperature may beobtained from another sensor within the same environment as the massflow controller: one located at a workstation, for example.

The controller 400 includes a local user interface 416 that may be usedwith one or more input devices, such as a keypad, keyboard, mouse,trackball, joy stick, buttons, touch screens, dual inline packaged (DIP)or thumb-wheel switches, for example, to accept input from users, suchas technicians who operate a mass flow controller. The local userinterface 414 may also include one or more outputs suitable for drivingone or more devices, such as a display, which may be an indicator light,a character, alphanumeric, or graphic display, or an audio output deviceused to communicate information from a mass flow controller to a user,for example. A communications interface 416 permits a mass flowcontroller to communicate with one or more other instruments, and/orwith a local controller, such as a workstation that controls a tool thatemploys a plurality of mass flow controllers and/or other devices in theproduction of integrated circuits, for example.

In this illustrative example, the communications interface 414 includesa DeviceNet interface. DeviceNet is known and discussed, for example, inU.S. Pat. No. 6,343,617 B1 issued to Tinsley et al. Feb. 5, 2002, whichis hereby incorporated by reference. The controller 400 also includesstorage 418 in the form, for example, of electrically erasableprogrammable read only memory (EEPROM) that may be used to storecalibration data, mass flow controller identification, or code foroperating the mass flow controller, for example. Various other forms ofstorage, such as random access memory (RAM), may be employed. Thestorage can take many forms, and, for example, may be distributed, withportions physically located on a controller “chip” (integrated circuit)and other portions located off-chip. The controller 400 employs a dataprocessor 420, which might take the form of an arithmetic logic unit(ALU) in a general purpose microprocessor, for example, to reduce data.For example, the data processor 420 may average readings received at thesensor inputs, determine the number of times a sensor reading hasexceeded one or more threshold values, record the time a sensor readingremains beyond a threshold value, or perform other forms of datalogging.

Pressure transients on the inlet supply line to a mass flow controllerthat employs a thermal mass flow sensor may create erroneous mass flowreadings. Erroneous mass flow readings may lead, in turn, to impropercontrol of a mass flow controller's outlet valve, which could damage ordestroy articles being processed with gasses under control of the massflow controller. The digital representation of mass flow may take theform of one or more data values and is subject to fluctuations due topressure transients on the inlet line of the mass flow sensor. In anillustrative embodiment, the controller 400 employs data obtained at thepressure sensor interface 410 to compensate for fluctuations induced ina thermal mass flow sensor by pressure transients on the mass flowsensor inlet line. In this illustrative embodiment, the controller 400obtains temperature information through a temperature interface 411. Thecontroller 400 employs the temperature, pressure, and mass flow readingsobtained from the respective interfaces, to produce a compensated massflow reading that more closely reflects the mass flow at the outlet ofthe mass flow sensor than a reading from the thermal mass flow sensoralone provides. The controller 400 also provides control to sensors, asnecessary, through flow sensor interface, pressure sensor interface, andtemperature interfaces, 408, 410, and 411, respectively.

The controller 400 also includes a valve actuator interface 404, whichthe controller 400 employs to control the position of a valve, such asthe valve 220 of FIG. 2, to thereby control the rate of fluid flowthrough a mass flow controller, such as the mass flow controller 200, ina closed-loop control process. The valve actuator may be asolenoid-driven actuator or piezoelectric actuator, for example. Thecontroller 400 must be capable of operating with sufficient speed toread the various sensor outputs, compensate as necessary, and adjust themass flow controller outlet control valve to produce a predeterminedflow rate. The flow rate is predetermined in the sense that it is“desired” in some sense. It is not predetermined in the sense that itmust be a static setting. That is, the predetermined flow rate may beset by an operator using a mechanical means, such as a dial setting, ormay be downloaded from another controller, such as a workstation, forexample, and may be updated.

In an illustrative embodiment, the controller 400 employs readings fromthe pressure interface 410 to compensate flow measurements obtained atthe mass flow interface 408 from a thermal mass flow sensor that sensesmass flow at the inlet to a mass flow controller, such as sensor 204,inlet 211 and controller 200. The compensated flow measurement moreaccurately depicts the flow at the outlet such as outlet 222 of the massflow controller 200. This outlet flow is the flow being directlycontrolled by the mass flow controller 200 and typically is the flow ofinterest to end users. Employing a pressure-compensated flow measurementin accordance with the principles of the present invention improves theaccuracy of a mass flow sensor's outlet flow reading and thereby permitsa mass flow controller to more accurately control the flow of fluids.That is, at equilibrium, mass flow at a mass flow controller's inlet isequal to the mass flow at the outlet of the mass flow controller, butduring inlet or outlet pressure transients, the flow rates differ,sometimes significantly. As a result, a mass flow controller thatprovides closed loop control using its inlet flow to control its outletflow may commit substantial control errors.

The steady state mass flow in the capillary sensor tube 320 of a thermalmass flow sensor 204 such as described in the discussion related to FIG.3 is generally described by the following equation:

$\begin{matrix}{Q_{c} = {\frac{d_{c}^{2}}{32\mu}\frac{\rho_{i}}{\rho_{R}}\left( \frac{P_{i} - P}{L_{c}} \right)}} & (1)\end{matrix}$

-   -   where:    -   d_(c)=capillary tube inside diameter    -   L_(c)=capillary tube length    -   ρ_(i)=the density of the gas at the inlet    -   ρ_(R)=the density of the gas at standard temperature and        pressure    -   μ=the gas viscosity    -   Pi=the pressure at the inlet of the mass flow controller    -   P=the pressure in the dead volume of the mass flow controller

The total flow through the mass flow controller is related to thatthrough the capillary sensor tube 320 through a split ratio:

α≡QBP/Qc

-   -   where QBP is the flow through the bypass channel 216 and Qc is        the flow through the capillary tube 320. The total flow Qi at        the mass flow controller inlet 214 is:

Qi=QBP+Qc=(1+α)Qc

If flow remains laminar in both the bypass and capillary, the splitratio will remain constant. When the inlet pressure varies with time,the nature of the inlet pressure transient and the pressurization of thedead volume govern the flow at the inlet. Assuming that allthermodynamic events within the dead volume occur at a constanttemperature that is equal to the temperature of the enclosure that formsa partial receptacle around the dead volume, the mass conservationwithin the dead volume may be described by:

$\begin{matrix}{Q_{o} = {Q_{i} - {\frac{T_{R}V}{T_{w}P_{R}}\frac{P}{t}}}} & (2)\end{matrix}$

-   -   Where:    -   P_(R)=pressure at standard temperature and pressure (760 Torr.)    -   T_(R)=temperature at standard temperature and pressure (273 K)    -   T_(w)=wall temperature (temperature of the wall of the mass flow        controller)    -   V=volume of the dead volume    -   Q_(i)=inlet flow to the mass flow controller    -   Q_(o)=outlet flow from the mass flow controller

A mass flow sensor in accordance with the principles of the presentinvention employs the relationship of equation (2) to compensate athermal mass flow sensor's mass flow signal and to thereby substantiallyreduce errors in mass flow readings during pressure transients.

The flow chart of FIG. 5 depicts the process of compensating a thermalmass flow sensor reading in accordance with the principles of thepresent invention. The process begins in step 500 and proceeds fromthere to step 502 where a mass flow sensor's controller, such as thecontroller 400 of FIG. 4, obtains a mass flow reading. This reading maybe obtained from a thermal mass flow sensor through a flow interface,such as interface 408 of FIG. 4, for example. This flow measurementreflects the rate of mass flow at the inlet of a mass flow controllerand, as previously described, may not adequately represent the mass flowrate at the outlet of the mass flow controller. The mass flow rate atthe outlet of a mass flow controller is generally the rate of interestfor use in control applications. Consequently, a mass flow controller inaccordance with the principles of the present invention compensates forthe inaccuracy inherent in assuming that the inlet flow rate to a massflow controller is equal to the outlet flow rate from the mass flowcontroller. From step 502 the process proceeds to step 504 where thesensor controller 400 obtains the temperature of the flow in the bypasschannel. The temperature could be obtained through a temperatureinterface such as interface 412 of FIG. 4, or it may be downloaded tothe compensated mass flow sensor. The compensation process may safelyassume that the gas temperature is equal to the temperature of theenclosure of the mass flow controller. Additionally, in mostapplications, the temperature will remain relatively stable over a longperiod of time, so that a stored temperature value may be employed, withupdates as necessary.

After obtaining the gas temperature in step 504 the process proceeds tostep 506 where the sensor controller 400 obtains the volume of the deadvolume 216 a. This value may have been stored during manufacturing, forexample. From step 506 the process proceeds to step 508 where thepressure within the dead volume 216 a is obtained over a period of time.The number of measurements and the time over which the measurements aremade depend upon the speed and duration of transients at the inlet ofthe mass flow controller. In step 510 the processor employs the pressuremeasurements made in step 508 to compute the time rate of change ofpressure within the dead volume. After computing the time rate of changeof pressure within the dead volume, the process proceeds to step 512where a compensated outlet flow value is computed according to equation(2). Simplifications may be made in the computational process. Forexample, the volume of the dead volume, standard temperature, andstandard pressure may all be combined into a single constant for usewith the inlet flow measurement and time rate of change of pressurewithin the dead volume to compute a compensated outlet flowapproximation. This simplification would yield an equation of the form:

Qo=Qi−C ₁(V/T)(dP/dt)  (3)

-   -   where:    -   Qo=the compensated sensed outlet flow rate,    -   Qi=the sensed inlet flow rate,    -   C₁=a normalizing constant relating the temperature and pressure        to standard temperature and pressure    -   V=the volume between the sensor bypass and the outlet flow        control valve,    -   T=the temperature of the fluid within the volume,    -   dP/dt=time rate of change of pressure within the volume.

As previously noted, the volume V could be folded into the constant C₁.From step 512 the process proceeds to step 514 where it continues, withthe flow sensor's controller obtaining pressure, temperature, and flowreadings and computing a compensated outlet flow estimate Qo, asdescribed. The process proceeds from step 514 to end in step 516, forexample, when the mass flow sensor is shut down.

Returning to the block diagram of FIG. 4, in this illustrativeembodiment, the controller 400 includes a diagnostic interface 422 thatpermits an operator, such as a technician for example, to not onlyinitiate, but conduct diagnostic tests on the mass flow controller.Furthermore, the interface 422 permits the operator to conduct thediagnostics in a manner that requires no input from the local systemcontroller, which may be a workstation, that otherwise normally controlsthe mass flow controller. Such diagnostics are transparent to the localsystem controller, which may not even be made aware of the diagnosticsbeing performed and may, consequently, continue its operations unabated.The diagnostic interface provides access to mass flow controller sensormeasurements, control outputs and mass flow controller diagnostic inputsand outputs. These various inputs and outputs may be exercised andmeasured through the diagnostic interface with very little delay. In anillustrative dual-processor embodiment described in greater detail inthe description related to the discussion of FIG. 9, a deterministicprocessor may modify outputs and/or monitor inputs, from sensors or testpoints, for example. During the execution of on-line diagnostics, thecontroller continues to execute its process control functions,unimpeded, while, at the same time, the controller may provide real-timeinteraction with a technician (i.e., interactions wherein the delays areimperceptible to a human operator) either locally or through atelecommunications connection.

Using the diagnostic interface 422, an operator can adjust controlvalues, such as the set point, used to determine the mass flowcontroller's operation. Additionally, the operator may modify sensoroutput values in order to test the mass flow controller's response tospecified sensor readings. That is, an operator can modify the sensorreadings a mass flow controller employs to control the flow of gassesthrough its outlet valve and, thereby, exercise the controller fordiagnostic purposes. An operator may read all sensor and test pointinputs as well as information stored regarding control (stored by thedeterministic controller in the dual processor embodiment), read allsensor values, read test point values, read control information, such asthe desired set point. Additionally, the operator may write to controloutputs and test points and over-write stored values, such as sensorreadings or set point information in order to fully test the controllerthrough the diagnostic port.

In an illustrative embodiment, a mass flow controller in accordance withthe principles of the present invention may include a web server. Such aweb server may be included within the diagnostic interface 422, forexample. In such an embodiment, the diagnostic interface 422 includes aweb-server that permits the mass flow controller to be used in a systemsuch as illustrated in the block diagram of FIG. 6. In such a system, auser, such as a technician, may employ a web-enabled device 600 such asa personal computer, personal digital assistant, or cellular telephonethat runs a web browser (e.g., Netscape or Explorer) to communicate witha server 602 embedded in the mass flow controller 604. The server 602includes web pages that provide an interface for the user to the massflow controller 604 in accordance with the principles of the presentinvention. The discussion related to FIGS. 13A through 13E providegreater detail related to the web server capability embedded in anillustrative embodiment of a mass flow controller in accordance with theprinciples of the present invention.

Mass flow sensors are typically calibrated during their manufacturingprocess. Because a mass flow sensor is usually incorporated into a massflow controller, this discussion will center on mass flow controllers,but the methods and apparatus discussed herein are applicable to“standalone” mass flow sensors as well. The calibration process requiresa technician to supply a gas at a known flow rate to the mass flowcontroller and correlate the mass flow sensor's flow signal to the knownflow rate. For example, in the case of a mass flow sensor that providesa voltage output corresponding to flow, the technician maps the voltageoutput from the sensor into the actual flow rate. This process may berepeated for a plurality of flows in order to develop a set ofvoltage/flow correlations: for example, a 4 Volt output indicates a 40standard cubic centimeter per minute (sccm) flow, a 5 Volt outputindicates a 50 sccm flow, etc.

Flow rates that fall between calibration points may be interpolatedusing linear or polynomial interpretation techniques, for example. Thisprocess may be repeated for several gases. Correlation tables thatrelate the signal from the mass flow sensor (which may be a voltage) toflow rates for various gases may thus be developed and stored. Suchtables may be downloaded to a mass flow controller for use “in thefield”, or may be stored within a mass flow controller. Often,technicians calibrate a mass flow controller using a relativelyinnocuous gas, such as N₂, and provide calibration coefficients that maybe used to correlate the flow of another gas to the calibration gas.These calibration coefficients may then be used in the field when aknown gas is “flowed” through the mass flow controller to compute theactual flow from the apparent flow. That is, the apparent flow may be aflow correlated to N₂ and, if Arsine gas is sent through the mass flowcontroller, the mass flow controller multiplies the apparent flow by anArsine gas calibration coefficient to obtain the actual flow.Additionally, once in the field, mass flow controllers may bere-calibrated on a regular basis to accommodate “drift”, orientation,water content of a gas the flow of which is being controlled, or tocompensate for other factors. U.S. Pat. No. 6,332,348 B1, issued on Dec.25, 2001 to Yelverton et al., which is hereby incorporated by reference,discusses these factors, and the unwieldy processes and equipmentrequired to carry out these in-the-field calibrations in greater detail.

A calibration method and apparatus in accordance with the principles ofthe present invention will be described in the discussion related to theconceptual block diagram of FIG. 7. This calibration system and methodmay be employed in a manufacturing setting, or, in an illustrativeembodiment, may be incorporated into a self-calibrating mass flowcontroller. The mass flow controller 700 includes a mass flow sensor 702and an electronic controller 704 that receives a flow signal from themass flow sensor 702. A calibrator 706 includes a variable flow gassource 708, a receptacle of predetermined volume 710, and a pressuredifferentiator 712. It should be noted that the lines separatingdifferent functional blocks are somewhat fluid. That is, in differentembodiments, the function associated with one block may be subsumed byone or more other blocks. For example, in an illustrative embodiment,the pressure differentiator 712 is implemented all, or in part, by theexecution of code within the electronic controller 704. The variableflow gas source 708 provides a gas at proportional rates to both thereceptacle of predetermined volume 710 and the mass flow sensor 702. Theflow rate to the mass flow sensor 702 may be equal to the flow rate tothe receptacle of predetermined volume 710: i.e., a proportionalityconstant of 1, for example. The mass flow sensor 702 is configured toproduce a mass flow signal indicative of the flow that it senses and, inthis illustrative embodiment, this signal is sent to the electroniccontroller 704. The pressure differentiator 712 produces a signalcorrelated to the flow from the variable flow source 708 into thereceptacle of predetermined volume 710 according to the relationship ofequation 4:

Qo=Qi−C1(V/T)(dP/dt)  (4)

where:

Qo=the outlet flow rate in standard cubic centimeters per minute,

Qi=the inlet flow rate in standard cubic centimeters per minute,

C1=a normalizing constant relating the temperature and pressure tostandard temperature and pressure

V=the predetermined volume of the receptacle in liters,

T=the Kelvin temperature of the fluid within the receptacle,

dP/dt=time rate of change of pressure within the receptacle inTorr/second.

In an illustrative embodiment, the receptacle is closed and gas flowsinto the receptacle until the pressure within the receptacle equals thatof gas supplied by the variable flow source 708. In such an illustrativeembodiment, the variable flow source may be a constant-pressure sourcethat, as pressure within the receptacle builds, supplies gas at anexponentially decreasing flow rate. In such a case, the outlet flowQo=0, and the inlet flow, Qi is given by:

Qi=C1(V/T)(dP/dt)  (5)

The pressure differentiator 712 takes the time derivative of thepressure within the receptacle 710 and, given the normalizing constantC1, the predetermined volume V, and the gas temperature within thereceptacle, the differentiator (and/or the electronic controller 704)may determine the actual flow into the receptacle 710. Because the flowinto the receptacle is proportional to the flow into the thermal massflow sensor 702, the actual flow into the thermal mass flow sensor 702may also be determined by a multiplying the actual flow into thereceptacle by a proportionality constant (e.g., the proportionalityconstant is 1 if the flows are equal). The signal from the mass flowsensor is then correlated, by the electronic controller 704 for example,to the actual flow, determined as just described. Such correlationrelates one or more signal levels from the mass flow sensor to theactual flows. The pressure differentiator 712 may include analogdifferentiator circuitry, for example, that takes the time derivative ofthe pressure signal. The differentiator output signal, a signalrepresentative of the time derivative of the pressure within thereceptacle dP/dt, may be sampled by an analog-to-digital converter topermit the electronic controller 704, which may include amicroprocessor, DSP chip, or dual processors, for example to operate onthe time derivative signal. Alternatively, the pressure differentiator712 may convert the pressure signal to digital form for processing bythe electronic controller 704, which takes the time derivative of thepressure signal. In such an embodiment, the electronic controller 704,in combination with differentiator code, operates as the differentiator.The electronic controller 704 employs at least two pressure differencesdivided by corresponding time intervals to compute the derivative. Thegas may be supplied in parallel to the receptacle and mass flow sensor702, or it may be supplied in series, as will be described in greaterdetail in the following discussion related to a self-calibrating massflow controller.

In operation, a mass flow controller may be calibrated as justdescribed, using a plurality of gases, with the correlation values(mappings of sensor output to actual flow) stored in tables. Calibrationcoefficients, relating flow measurements of one gas to another may alsobe developed and stored. The tables and/or coefficients may bedownloaded to a mass flow controller in the field for use by thecontroller in controlling the flow of a gas. Various known interpolationtechniques, such as linear or polynomial interpolation may be employedin conjunction with the calibration tables and/or coefficients.Additionally, such stored calibration tables and/or coefficients may beused as default values in a self-calibrating mass flow controller inaccordance with the principles of the present invention. Aself-calibrating mass flow controller in accordance with the principlesof the present invention includes a calibrator 706 and a mass flowsensor 702 which may be employed to calibrate the mass flow controllerin a manner as just described. In the case of a self-calibrating massflow controller, though, the calibration can be performed, In Situ, inthe field just as readily as in a manufacturing setting.

Once installed in the field, on a semiconductor processing tool as inthe system 100 of FIG. 1, for example, the mass flow controller cancalibrate itself using the gas that is to be used during thesemiconductor processing. By using the gas that is to be used inprocessing, the mass flow controller may provide a more accurate flowmeasurement, because it will automatically accommodate variations, suchas moisture content, for example. Additionally, a new processing gas maybe used just as readily as a conventional gas, since theself-calibrating mass flow controller may calibrate itself (that is,correlate mass flow signal levels to actual flow levels determined bythe pressure differentiator), on the gas to be used, not in relation toanother, standard gas, such as N₂. Because the mass flow controller iscalibrated in the orientation in which it will be used, discrepanciesdue to re-orientation of the mass flow controller in the field relativeto the position in which it was calibrated during manufacturing will besubstantially eliminated. All the mass flow controllers within a systemsuch as system 100 of FIG. 1 may be calibrated automatically andsimultaneously, within moments. This is in contrast to the cumbersome,painstaking process employed for conventional mass flow controllers,which are typically individually calibrated by a technician employingmultiple mass flow meters, going from mass flow controller to mass flowcontroller. As will be described in greater detail in the descriptionrelated to the discussion of FIG. 8, a mass flow controller thatincludes a thermal mass flow sensor and a pressure transducer may shutits outlet valve to create a varying gas flow into it's dead volume. Bytaking the time derivative of the pressure the actual flow into the deadvolume receptacle may be determined. The mass flow controller'scorrelation of the actual value of the flow to the thermal mass flowsensor signal acts as the mass flow controller's calibration.

FIG. 8 is a conceptual block diagram of a self-calibrating mass flowcontroller 800 in accordance with the principles of the presentinvention. In this illustrative, series-flow embodiment, a gas flowsthrough a thermal sensor 802 into a receptacle of predetermined volume804, then through an outlet valve 806. The outlet flow Qo would normallybe a controlled flow into a chamber, such as a chamber within anintegrated circuit processing tool. An electronic controller 808, which,in this illustrative embodiment, executes code to perform thedifferentiation required to obtain actual flow, as described in thediscussion related to FIG. 7, is in communication with the thermalsensor 802, pressure sensor 805 and the outlet valve 806. In anillustrative process, the electronic controller 808 operates inconjunction with the outlet valve 806 to form a variable-flow gassupply. That is, the electronic controller shuts the outlet valve, whichcauses the flow to decrease exponentially. The pressure within the deadvolume increases, and the electronic controller differentiates thissignal a number of times in order to obtain actual flow readings tocorrelate to the mass flow sensor signal values over a relatively broadrange of flows. Additionally, in order to extend the period of timeduring which the flow is varying and to obtain actual flow values forcorrelation with the thermal mass flow signal values over a broad range,the electronic controller may open the outlet valve to a fully openposition before closing it.

The pressure and flow profiles associated with such a process areillustrated conceptually in the graph of FIG. 9. At an initial time t₀,the pressure difference between gas at the inlet to the mass flowcontroller Pin and the pressure Pr downstream in the receptacle 804forces gas to flow through the mass flow controller at a rate Qin. Inthis example, the inlet pressure P_(in), pressure within the receptacleP_(R), and flow through the input of the mass flow controller areconstant. At time t_(so), the controller shuts the outlet valve, therebyreducing outlet flow Qo to zero. Gas continues to flow into thereceptacle as long as there is a pressure difference between thereceptacle and the inlet. As the pressure P_(r) within the receptaclerises exponentially toward an equilibrium state of equality with theinlet pressure P_(in), the inlet flow Q_(in) decreases. By taking thederivative of the pressure change within the receptacle (also referredto herein as “dead volume” in association with an illustrativeembodiment of the invention), the electronic controller may determinethe actual flow into the receptacle, as previously described.

The electronic controller may correlate a plurality of simultaneousreadings produced by the thermal mass flow sensor 802, to therebycalibrate the mass flow sensor 802. That is, once this process iscompleted for a specific gas, flow readings from the thermal mass flowsensor 802 may be correlated to actual flow rates. The results may beemployed by the electronic controller 808 to control the opening of thevalve 806 in a closed loop control system in order to deliver a selectedflow downstream. In order to increase the period of time t_(so) fromwhen the controller shuts the valve, to the time at which the flowbecomes undetectable, and to thereby increase the number and precisionof pressure measurements that may be made, the controller may open thevalve completely before shutting it at time t_(so). Additionally, one ormore flow restrictors may be placed in the flow path between the inletto the thermal mass flow sensor 802 and the inlet to the receptacle 804.

The conceptual block diagram of FIG. 10 illustrates the architecture ofa dual-processor embodiment of an electronic controller 1000 such as maybe used in a mass flow sensor in accordance with the principles of thepresent invention. In this illustrative embodiment, the controllerincludes two processors 1002, 1004. One of the processors 1002 isdedicated to “real time” processes and the other processor 1004 isdedicated to non-real time processes. By “real time” we mean processesthat require a specified level of service within a bounded responsetime. In this sense, the processes are deterministic and the processor1002 will be referred to herein as the deterministic processor. Theobjective of the dual processor architecture is to reduce the number ofinterrupts and manage asynchronous event responses in a predictable way.The non-deterministic processor 1004 may handle event-driven interrupts,such as responding to input from a user. The deterministic processor1002 handles only frame-driven, that is, regularly scheduled,interrupts. In an illustrative embodiment, the non-deterministicprocessor 1004 is a general purpose processor, suited for a variety oftasks, such as user-interface, and other, miscellaneous tasks, ratherthan a specialized co-processor, such as a math- orcommunications—coprocessor. In particular, a TMS320VC5471, availablefrom Texas Instruments, Inc., may be employed in a dual-processorembodiment in accordance with the principles. The TMS320VC5471 isdescribed in the related data manual available from Texas Instruments,Inc., which data manual is hereby incorporated by reference.

A processor interface 1006 provides for inter-processor communications.The deterministic processor 1002, includes sensor and actuatorinterfaces. Among the sensor interfaces, a flow sensor interface 1005operates in conjunction with a mass flow sensor to produce a digitalrepresentation of the rate of mass flow in an associated mass flowcontroller. One or more actuator interfaces 1010 are employed by thedeterministic processor 1002 to control the opening of an associatedmass flow controller's output control valve or drive a diagnostic testpoint, for example. The actuator may be a current-driven solenoid or avoltage-driven piezo-electric actuator, for example. As will bedescribed in greater detail in the discussion related to the flow chartof FIG. 9, after initialization, the deterministic processor 1002 loopsthrough a control sequence, gathering sensor data, gathering settinginformation (for example, a desired mass flow setting), providing statusinformation, and controlling the state of the outlet valve. Becausenon-deterministic tasks are offloaded to the non-deterministic processor1004, the deterministic processor's control loop may be very compact.Consequently, control tasks may be executed within a minimal period oftime and control readings and drive signals may be updated morefrequently than possible if time were set aside for servicingnon-deterministic tasks

The controller 1000 operates in conjunction with a thermal mass flowsensor as generally described in the discussion related to FIG. 3 toproduce a digital representation of the rate of mass flow into anassociated mass flow controller. The digital representation may take theform of one or more data values and is subject to fluctuations due topressure transients at the input of the mass flow sensor. The controller1000, and more specifically, the deterministic processor 1002 may employdata obtained at the pressure sensor interface 1006 to compensate forfluctuations induced in the thermal mass flow sensor by pressuretransients on the mass flow sensor inlet line. In this illustrativeembodiment, the deterministic processor 1002 employs the temperature,pressure, and mass flow readings obtained from the respective 1008,1007, and 1005 interfaces, to produce a compensated mass flow readingthat more closely reflects the mass flow at the outlet of the mass flowsensor than a reading from the thermal mass flow sensor alone. Thedeterministic processor 1002 also provides control to sensors, asnecessary, through thermal flow 1005, pressure 1007, and temperature1008 sensor interfaces. The compensation process will be described ingreater detail in the discussion related to FIG. 11. The deterministicprocessor 1002 also includes a valve actuator interface 1010, which thedeterministic processor employs to control the position of a valve, suchas the valve 220 of FIG. 2, to thereby control the rate of fluid flowthrough a mass flow controller, such as the mass flow controller 200, ina closed-loop control process.

The deterministic processor 1002 is devoted to the closed-loop valvecontrol process, and, consequently, must be capable of operating withsufficient speed to read the various sensor outputs, compensate asnecessary, and adjust the valve to produce a predetermined flow rate.The flow rate is predetermined in the sense that it is “desired” in somesense, and it need not be a static setting. That is, the predeterminedflow rate may be set by an operator using a mechanical means, such as adial setting, or may be downloaded from another controller, such as aworkstation, for example, and updated frequently. Typically, gas flowcontrol, and in this case, compensated gas flow control, requiresrelatively high-speed operation. Various types of processors, such asreduced instruction set (RISC), math coprocessor, or digital signalprocessors (DSPs) may be suitable for such high-speed operation. Thecomputational, signal conditioning, and interfacing capabilities of aDSP make it particularly suitable for operation as the deterministicprocessor 1002. As will be described in greater detail in thedescription of the control process related to the discussion of FIG. 9,the function performed by the deterministic processor 1002 isdeterministic in the sense that certain operations are completed in atimely and regular manner in order to avoid errors, and possibleinstabilities, in the control process. The deterministic 1002 andnon-deterministic 1004 processors communicate via the inter-processorinterface 1006 in a manner that does not impede the deterministicoperation of the deterministic processor 1002.

The non-deterministic processor 1004 includes a local user interface1016 that may be used with one or more input devices, such as a keypad,keyboard, mouse, trackball, joy stick, buttons, touch screens, dualinline packaged (DIP) or thumb-wheel switches, for example, to acceptinput from users, such as technicians who operate a mass flow controllerassociated with the non-deterministic processor 1004. The local userinterface 1016 also includes one or more outputs suitable for drivingone or more devices, such as a display, which may be a character,alphanumeric, or graphic display, for example, indicator light, or audiooutput device used to communicate information from a mass flowcontroller to a user. A communications interface 1018 permits a massflow controller to communicate with one or more other instruments,and/or with a local controller, such as a workstation that controls atool that employs a plurality of mass flow controllers and/or otherdevices in the production of integrated circuits, for example. In thisillustrative example, the communications interface 1018 includes aDeviceNet interface. A diagnostic interface 1020 provides an interfacefor a technician to run diagnostics, as previously described in relationto the diagnostic interface 422 of FIG. 4. In an illustrativeembodiment, the diagnostic interface includes an Ethernet interface anda web server.

The compactness of code for the deterministic processor 1002 permits thedeterministic processor to be highly responsive to input changes and toquickly modify actuator signals in response to those changes. Thispartitioning of operations between deterministic and non-deterministicprocessors also eases the initial development of code, for both thedeterministic and non-deterministic processors. For example, thedeterministic code needn't respond to unscheduled events, such a as“mirroring” a user's requests on a display at a user interface, and thenon-deterministic code needn't break away from providing such userfeedback in order to adjust an outlet valve control setting every fiftybus cycles. The partitioning between deterministic and non-deterministicalso permits relatively simple revisions and upgrades. If the code forone processor must be revised or upgraded, the code for the other mayrequire no revisions or only minor revisions. In particular, the codefor the deterministic processor may be more “mature”, or fixed than thatfor the non-deterministic processor; user interfaces, communications andother similar functions tend to be upgraded more frequently than thedeterministic, mass flow control, functions.

Using this illustrative dual-processor embodiment, a user interface maybe updated without any impact on the control function code, for example.Revision and maintenance of mixed-mode code (deterministic andnon-deterministic code) would be a much more complicated and costlyproposition than code partitioned in a manner in accordance with theprinciples of the present invention. In an illustrative embodiment thedual-processor controller 1000 may by a hybrid processor thatincorporates two processors on one integrates circuit. An integratedcircuit such as the TMS320C5471 hybrid processor available from TexasInstruments may be employed as the dual processors in accordance withthe principles of the present invention. The digital signal processing(DSP) subsystem of the chip, due to its math capabilities would be moresuitable as the deterministic processor in such an application. The IC'sdual-ported memory may be employed as the inter-processor interface,with the processors writing to and reading from memory locations setaside to act as “mail boxes” for the transfer of information, includingdata, commands, and command responses.

The flow chart of FIG. 11 outlines the process of sensing andcontrolling the flow of gas through a dual-processor mass flowcontroller in accordance with the principles of the present invention.The process begins in step 1100 and proceeds from there to step 1102where the controller is initialized. This initialization step mayinclude the uploading of calibration values or a calibration sequenceitself. Additionally, operating code for both the deterministic andnon-deterministic processors may be uploaded at this point. In anillustrative embodiment, the non-deterministic processor 1004 may uploadits own code and begin operating, then upload code for the deterministicprocessor 1002. In the process of uploading code for the deterministicprocessor 1002, the non-deterministic processor 1004 may select among aplurality of executable code sets to upload to the deterministicprocessor, thereby tailoring the operation of the deterministicprocessor 1002. The non-deterministic processor 1004 may base thisselection on switch settings, commands from a local controller (e.g., aworkstation controlling the operation of a semiconductor process tool),or settings stored in non-volatile storage, for example. Such aselection permits a mass flow controller to be tailored to differentflow control operations. For example, a technician may, by selectingamong code sets, choose to operate the controller in a “pressurecontroller” mode rather than a “mass flow controller” mode, and thisselection may be made locally or remotely (i.e., through atelecommunications link).

In step 1104 the non-deterministic processor 1004 passes operating codeand initial control settings to the deterministic processor 1002 whichthen begins operating in a manner described generally in connection withthe flow chart of FIG. 12. From step 1104, the process proceeds to step1106 where the non-deterministic processor 1004 services the localinput/out interface. Such servicing may include reading various inputs,including keyboard, switch, or mouse inputs, and displaying informationlocally, through LEDS, alphanumeric displays, or graphical displays.From step 1106 the process proceeds to step 1108 where thenon-deterministic processor 1004 services the communications interface1018. This servicing may include the steps of uploading control andsensor data to a workstation that operates as the local controller of asemiconductor process tool, for example. Additionally, thenon-deterministic processor 1004 may download updated settings from thelocal controller.

From step 1108 the process proceeds to step 1110 where thenon-deterministic processor 1004 services the diagnostic interface 1020.Various diagnostic operations, such as set forth in the descriptionrelated to the discussion of FIG. 4, may be performed in this step. Inan illustrative embodiment, the mass flow controller includes a webserver, which permits an operator to run diagnostics through a networksuch as the “world wide web.” From step 1110 the process proceeds tostep 1112 where the non-deterministic processor 1004 services theinter-processor interface 1006. During “normal”, non-diagnosticoperation, the non-deterministic processor 1004 obtains readings fromthe deterministic processor 1002 and passes control information, such asa flow setting obtained through the communications interface 1018, tothe deterministic processor 1002. From step 1112, the process proceedsto continue the processes just set forth in step 1114. The processproceeds to end in step 1116 when the mass flow controller is turnedoff, for example.

As previously noted, the steps set forth in this and other flow chartsherein need not be sequential and, in fact, a number of functionsperformed by the non-deterministic processor 1004 may beevent-interrupt-driven and no predictable sequence may be ascribed tothe non-deterministic processor's operation. Other processes, such asdata-logging may be performed at regular intervals. Thenon-deterministic processor 1004 can support a two-way socket connectionto the deterministic processor 1002 through an Ethernet networkinterface, for example, to provide a relatively direct connectionbetween a remote user and the deterministic processor 1002.

The flow chart of FIGS. 12A-12B depicts the operation of thedeterministic processor 1002 of a dual processor mass flow controller inaccordance with the principles of the present invention. In the contextof this flow chart, it is assumed that an initialization process hastaken place and that the deterministic processor 1002 is cycling throughits control loop. The process begins in step 1200, FIG. 12A, andproceeds from there to step 1202 where the deterministic processordetermines whether it is to operate in its “normal” control capacity orwhether it is to operate in an alternative mode, such as a manualdiagnostic mode or an automatic diagnostic mode, for example. Thedeterministic processor 1002 bases this decision on information itobtains from the inter-processor interface 1006. The deterministicprocessor 1002 services frame-driven, rather than event-driveninterrupts; consequently, it regularly polls the inter-processorinterface 1006 to obtain information such as this.

If the deterministic processor is to operate in its normal mode, theprocess proceeds from step 1202 to step 1204, where the deterministicprocessor obtains information from the inter-processor interfaceregarding the desired control settings. This information may be in theform of a desired flow rate received from a local controller, from afront panel user interface, or through the diagnostic port 1020 forexample. The deterministic processor may also transfer information, suchas sensor data, for example, to the non-deterministic processor throughthe inter-processor interface during this step. From step 1204 theprocess proceeds to step 1206 where the deterministic processor gathersdata, from a variety or sensors for example. The sensors from which thedeterministic processor obtains data may include a mass flow sensor(thermal or other type), a temperature sensor, or a pressure sensor, forexample.

From step 1206 the process proceeds to step 1208, where thedeterministic processor 1002 computes the flow rate of material throughthe mass flow controller. In an illustrative embodiment, the mass flowcontroller includes a thermal mass flow sensor and a pressure sensorconfigured to measure the pressure within the dead volume between thethermal mass flow sensor's bypass and the mass flow controller outletvalve. In this embodiment, the deterministic processor may employ themethod described in relation to the discussion of FIG. 5 to compensate aflow rate measured by a thermal mass flow sensor at the inlet of thecontroller to more closely approximate the flow rate at the outlet ofthe controller. In an embodiment in which the flow rate obtained fromthe sensor is not compensated, the process would proceed directly fromstep 1206 to step 1210, skipping the computational process of step 1208.

In step 1210 the deterministic processor 1002 determines whether theflow rate computed in step 1208 (or read in step 1206) is equal to thedesired flow rate indicated by the setting information obtained from thenon-deterministic processor 1004 via the inter-processor interface 1006in step 1204. If the values are equal, the deterministic processorcontinues the operation as just described, as indicated by the“continue” block 1214 (i.e., the deterministic processor returns to step1202 and continues to cycle through the loop). If the values are notequal, the deterministic processor computes an error signal and employsthe error signal to adjust the drive signal to the mass flowcontroller's outlet valve at step 1212. From step 1212 the processproceeds to continue in step 1214. The process proceeds from step 1214to end in step 1216 when the mass flow controller is shut down or reset,for example.

If, in step 1202 the deterministic processor concludes that it is not tooperate in the normal mode, the process proceeds through connecting boxA to step 1218, FIG. 12B, where the deterministic processor 1002determines whether it is to operate in a diagnostic mode. Thedeterministic processor 1002 may obtain this information from theinter-processor interface 1006. If the deterministic processor is tooperate in a diagnostic mode, the process proceeds to step 1220. In step1220 the deterministic processor determines which diagnostic mode it isto operate in. Once again, this information may be passed to thedeterministic processor through the inter-processor interface. In an“automatic” mode, the deterministic processor acquires a sequence ofdiagnostic values from the inter-processor interface. The sequence ofvalues is available at the interface for acquisition by thedeterministic processor. The diagnostic values may be control outputs,for setting the opening of the mass flow controller outlet valve or forsetting test point drive values, for example, or the diagnostic valuesmay indicate desired sensor readings or readings from test points, forexample. The diagnostic values may also indicate the sequence in whichthe values are to be employed, in order to set test point driver values,then read test point outputs, for example. In a manual mode, diagnosticvalues are made available to the deterministic processor through theinter-processor interface one at a time. In an embodiment in which themass flow controller includes a web server, a technician may use aweb-enabled workstation to contact the server in the mass flowcontroller. Once linked to the server, the technician may enter a valvesetting command, by typing, selecting from a pull down menu or clickingon icon, for example. This single, setting, command would be received bythe non-deterministic processor 1004 through its diagnostic port andpassed to the deterministic processor 1002 through the inter-processorinterface 1006.

In the manual diagnostic mode the deterministic processor executesthrough whatever diagnostic values are available at the inter-processorinterface, then returns to it's normal control loop. This could“override” a single control loop cycle if, for example, a singlediagnostic value, such as a test point drive value, is presented to thedeterministic processor or, if a sequence of diagnostic values ispresented to the deterministic processor, a number of control loopcycles may be overridden. In the automatic diagnostic mode a number ofdiagnostic values may be exchanged through the inter-processor interfacein a period corresponding to a few control loop cycles, with asubstantial number, on the order of at lest ten times as many, controlloop cycles intervening between automatic diagnostic exchanges.Diagnostic modes may be combined, for example, to produce an automaticactive on-line diagnostic mode, for example. In an illustrativeembodiment, a mass flow controller in accordance with the principles ofthe present invention operates on a one-millisecond control loop cycle,during which it provides one percent of full-scale accuracy.

Keeping the various diagnostic modes in mind, and keeping in mind thatprocesses illustrated through the use of flow charts may not be strictlylinear processes and alternative flows may be implemented within thescope of the invention, the diagnostic process will be describedgenerally in relation to steps 1220 through 1226. In step 1220 thedeterministic processor acquires diagnostic values from theinter-processor interface. As previously noted, these values may be forthe deterministic processor to use as control outputs or they mayindicate data that is to be acquired by the deterministic processor,from a sensor, for example. From step 1220 the process proceeds to step1222 where the deterministic processor 1002 processes the valuesacquired in step 1220, by changing an outlet valve actuator drive signalor transferring a sensor reading to the inter-processor interface, forexample.

From step 1222 the process proceeds to step 1224 where the deterministicprocessor determines whether it has completed its diagnostic tasks. Ifit has not completed its diagnostic tasks, for example if it isoperating in the automatic diagnostic mode and there are more values ina sequence of values to be retrieved from the inter-processor interface,the process returns to step 1222 and from there as previously described.If, in step 1224 the deterministic processor concludes that it hascompleted its diagnostic task, the process returns through connectingbox B to step 1214 of FIG. 12A. If the deterministic processordetermines that it is not to operate in a diagnostic mode, the processproceeds from step 1218 where processor performs functions such asroutine background operations, then proceeds to return throughconnecting block B to step 1214 and from there as previously described.

The screen shots of FIGS. 13A through 13E illustrate a user interfacesuch as may be made available for access to a mass flow controller inaccordance with the principles of the present invention that includes aweb server interface, such as the interface 608 of FIG. 6. In anillustrative embodiment the mass flow controller includes a web server,such as the server 602 of FIG. 6. A user may employ the server locally,through a local controller, or remotely, from a web-enabled device, suchas the device 600 of FIG. 6. In this manner, the same user interface maybe employed for both remote and local interactions with the mass flowcontroller. Detailed information regarding a mass flow controller, suchas model number, range, and manufacturing setup parameters, may bedisplayed to a user and user-changeable setup parameters may bedisplayed as well. Different display techniques may be employed. Ifthere are only a limited number of acceptable values, they may bedisplayed and chosen from a pulldown menu, for example. As previouslydescribed, a user, such as a technician can change set point values,open or close a valve, or monitor flow output, for example, through thisinterface. Additionally, while the mass flow controller is operatingunder a process control application, a user may induce the server toplot and log parameter values obtained from the mass flow controller.

The screen shot of FIG. 13A illustrates the display a user may encounterwhen first accessing a mass flow controller in accordance with theprinciples of the present invention over the web. The display promptsthe user to choose a communications protocol through use of the pulldownwindow 1300. The “query devices” link 1302 allows the user to initiate aprocess whereby his browser attempts to locate all devices that itrecognizes.

Basic information may be downloaded through the server. Informationrelated to the mass flow controller are displayed in the screen of FIG.13B. Such screens may be expanded or collapsed. A user may choose toview information related to a subset of the displayed mass flowcontrollers. Based on the model number, serial number and internallystored codes, product specifications for the mass flow controller aredisplayed along with user-selectable parameters, which may be displayedin a list, for example. A user may employ this screen to downloadcalibration data to or from a mass flow controller and to entercalibration tables. A user may also alter set points through thisinterface and monitor the reported flow through the corresponding massflow controller. Additionally, a user may override settings and open orclose a mass flow controller's outlet control valve. Each mass flowcontroller's specifications may be viewed, as illustrated by the screenof FIG. 13C. Illustrative user-selectable parameters are displayed inthe screen shot of FIG. 13D and calibration data such as a user maydownload from a mass flow controller is illustrated in the screen shotof FIG. 13E.

A software implementation of the above described embodiment(s) maycomprise a series of computer instructions either fixed on a tangiblemedium, such as a computer readable media, e.g. diskette, CD-ROM, ROM,or fixed disc, or transmittable to a computer system, via a modem orother interface device, such as communications adapter connected to thenetwork over a medium. Medium can be either a tangible medium, includingbut not limited to, optical or analog communications lines, or may beimplemented with wireless techniques, including but not limited tomicrowave, infrared or other transmission techniques. The series ofcomputer instructions embodies all or part of the functionalitypreviously described herein with respect to the invention. Those skilledin the art will appreciate that such computer instructions can bewritten in a number of programming languages for use with many computerarchitectures or operating systems. Further, such instructions may bestored using any memory technology, present or future, including, butnot limited to, semiconductor, magnetic, optical or other memorydevices, or transmitted using any communications technology, present orfuture, including but not limited to optical, infrared, microwave, orother transmission technologies. It is contemplated that such a computerprogram product may be distributed as a removable media withaccompanying printed or electronic documentation, e.g., shrink wrappedsoftware, preloaded with a computer system, e.g., on system ROM or fixeddisc, or distributed from a server or electronic bulletin board over anetwork, e.g., the Internet or World Wide Web.

Although various exemplary embodiments of the invention have beendisclosed, it will be apparent to those skilled in the art that variouschanges and modifications can be made which will achieve some of theadvantages of the invention without departing from the spirit and scopeof the invention. It will be apparent to those reasonably skilled in theart that other components performing the same functions may be suitablysubstituted. Further, the methods of the invention may be achieved ineither all software implementations, using the appropriate object orprocessor instructions, or in hybrid implementations that utilize acombination of hardware logic, software logic and/or firmware to achievethe same results. Processes illustrated through the use of flow chartsmay not be strictly linear processes and alternative flows may beimplemented within the scope of the invention. The specificconfiguration of logic and/or instructions utilized to achieve aparticular function, as well as other modifications to the inventiveconcept are intended to be covered by the appended claims.

The foregoing description of specific embodiments of the invention hasbeen presented for the purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseforms disclosed, and many modifications and variations are possible inlight of the above teachings. The embodiments were chosen and describedto best explain the principles of the invention and its practicalapplication, and to thereby enable others skilled in the art to bestutilize the invention. It is intended that the scope of the invention belimited only by the claims appended hereto.

1. A system comprising a plurality of flow paths, each between a gasinlet and an outlet, the system further comprising: a mass flowcontroller for each of the flow paths, each mass flow controllerincluding a mass flow sensor for sensing the mass flow along therespective flow path and a control valve, responsive to a controlsignal, for controlling the mass flow through the corresponding outlet;a pressure sensor for each of the flow paths for measuring fluctuationsin pressure of gas along the respective flow path; and at least oneprocess controller for generating each control signal as a function ofthe sensed mass flow and measured fluctuations in pressure of gas so asto provide a substantially constant flow of gas at each of the outletsirrespective of fluctuations in pressure of gas at the respective inlet.2. A system according to claim 1, wherein each pressure sensor measuresfluctuations in pressure of gas along the respective flow path, upstreamfrom the corresponding control valve.
 3. A system according to claim 1,wherein each pressure sensor measures fluctuations in pressure of gas inthe dead space between the respective flow sensor and the correspondingcontrol valve.
 4. A system, defining a plurality of flow paths, eachbetween a gas inlet and an outlet, the system comprising: a mass flowcontroller for each of the flow paths, each mass flow controllerincluding a mass flow sensor for sensing the mass flow rate along therespective flow path and a control valve, responsive to a controlsignal, for controlling the mass flow through the corresponding outlet;a pressure sensor for each flow path for measuring fluctuations inpressure of gas along the corresponding flow path; and at least oneprocess controller for generating each control signal as a function ofthe sensed mass flow rate at the respective inlet and the measuredfluctuations in pressure of gas so as to compensate the flow ratemeasured by the sensor at the corresponding inlet to more closelyapproximate the flow rate at the outlet.
 5. A system according to claim4, wherein the pressure sensor measures fluctuations in pressure of gasalong the flow path, upstream from the control valve.
 6. A systemaccording to claim 5, wherein the pressure sensor measures fluctuationsin pressure of gas in the dead space between the flow sensor and thecontrol valve.
 7. A control system for controlling the flow of gas ineach of a plurality of flow paths, the control system comprising: a massflow controller for each of the flow paths constructed and arranged soas to receive fluid flowing from a inlet supply line, each mass flowcontroller including a thermal mass flow sensor and an outlet valve; apressure sensor for each of the flow paths; and at least one processcontroller including at least one pressure sensor interface for couplingthe controller to a respective pressure sensor, wherein the controlleremploys data obtained at the pressure sensor interface to compensate forfluctuations induced in the thermal mass flow sensor by pressuretransients in the inlet supply line.
 8. A control system according toclaim 7, wherein the pressure sensor and pressure interface with respectto each flow path are used to monitor the pressure within thecorresponding mass flow controller.
 9. A control system according toclaim 7, wherein each mass flow controller includes a dead volume, andthe corresponding pressure sensor and pressure interface are used tomonitor the pressure within the dead volume.
 10. A control systemaccording to claim 7, wherein the process controller includes a flowsensor interface for coupling the process controller to the thermal massflow sensor.
 11. A control system according to claim 10, furtherincluding a temperature sensor and a temperature sensor interface foreach flow path for obtaining data representative of the temperature ofthe fluid flowing in the corresponding thermal mass flow sensor.
 12. Acontrol system according to claim 11, wherein each mass flow sensorincludes an outlet and the process controller employs readings oftemperature, pressure and mass flow obtained from the respectivetemperature sensor interface, pressure interface and flow sensorinterface to produce a compensated mass flow reading that more closelyreflects the mass flow at the outlet of the mass flow sensor than areading from the thermal mass flow sensor alone provides.